Modern automotive engines typically utilize a catalytic converter to reduce the exhaust gas emissions produced by the engine. Such converters operate to chemically alter the exhaust gas composition produced by the engine to help meet various environmental regulations governing tailpipe emissions. When operating properly, modern catalytic converters can reduce the concentration of the three major undesirable exhaust gases--hydrocarbons, carbon monoxide, and various nitrous oxides--by about 90%.
Several methods exist in the art for determining the operability of a catalytic converter. One method involves directly measuring the efficiency of the converter by using exhaust gas emission sensors. Another method involves determining catalytic converter efficacy based largely on measures of the oxygen storage capability of the converter. A converter with reduced oxygen storage capability may, however, function with acceptable efficiency in treating exhaust gas. That is, the treated exhaust gas may meet emission requirements notwithstanding such reduced oxygen storage capability, particularly by effective use of oxygen components present in the exhaust gas. In such cases, an accurate catalytic converter efficiency test could be less directly related to oxygen storage capability.
Another method, as proposed in SAE Paper No. 900062, Detection of Catalyst Performance Using On-Board Diagnostics, employs two exhaust gas oxygen (EGO) sensors, one upstream and one downstream of the catalytic converter, to detect oxygen content in the exhaust gas. The system employs test signals in the form of an air/fuel ratio swing on both sides of stoichiometry at predetermined rates or frequencies caused by fuel control system perturbations. A determination of catalytic converter efficacy is said to be made by comparing the difference in response patterns between the upstream and downstream EGO sensors. In such a system, the monitoring results depend on signals from two EGO sensors which may have different characteristics, due either to manufacturing tolerances or to differential aging over a period of use. Moreover, such a system requires the use of multiple EGO sensors.
Accordingly, there is a need for a method of determining the operability of a catalytic converter via on-board vehicle diagnostics in a reliable and accurate manner.